This invention relates to a ground detection arrangement for an A.C. generator. FIG. 1 illustrates an example of a conventional arrangement of the type of the present invention. In the FIG. 1, numeral 1 denotes a brushless A.C. generator, numeral 2 an armature winding, numeral 3 a three-phase full-wave rectifier, numeral 4 a field winding and numeral 5 an insulation resistance detector. The insulation resistance detector 5 includes both a D.C. voltage source (E) 8 and current relay 9 for the direct current introduced across a brush 6 and a slip ring 7 in between the neutral point C of armature winding 2 and the ground A. If the ground resistance (insulation resistance) R.sub.s is reduced to cause a current exceeding the critical reference value, the relay 9 is actuated. Numeral 10 denotes a protection resistor.
In such a conventional device, however, the current value I.sub.sp in the case of the grounding occurring on the side of the positive pole of field winding 4 and the current value I.sub.sn in the case of the grounding occurring on the side of the negative pole N are different from each other as follow: ##EQU1## where: E.sub.f =Voltage of field winding
R.sub.i =Resistance value of protection resistor PA1 Therefore, conventional devices do not allow for correct detection of the existence of grounding in the field winding 4.